The invention relates to a receiving method which is used in a radio system comprising at least one subscriber terminal and a base station, which in turn comprise a transmitter and a receiver receiving a signal group comprising signals that have propagated on their own channels and are filtered in the receiver to generate separate channel specific signals.
The invention also relates to a receiver arranged to be used in a radio system comprising a subscriber terminal and a base station, which in turn comprise a transmitter and a receiver arranged to receive a signal group comprising signals propagated on their own channels, and said receiver comprising a channel filter filtering the signal group to generate channel specific signals.
In a typical radio system, such as the GSM (Global System for Mobile Communication), a base station receives a number of subscriber terminal signals of different strengths and transmitted on different channels. Since these signals are the radio system""s own signals, they are typically rather narrowband signals with a restricted power. However, especially at the beginning of call set-up when power control is not yet functioning, the subscriber terminal often uses unnecessarily high power, in which case the linear range of operation of signal processing circuits at the base station is exceeded and the signal becomes distorted.
As digital signal processing components develop, an ever increasing number of functions in digital radio systems are performed digitally. A typical example of such a signal processing operation is digital channel filtering of a receiver in the GSM system. The remaining analog signal processing, which includes e.g. amplification, frequency conversion and filtering, is to be performed on the reception band in its entire width. It is also expedient to perform the A/D conversion required on a broad band basis, only in this case a problem arises with the linear range of operation of the A/D converter, which is too narrow as compared with the spreading of the power levels of the received signals, and the subsequent signal distortion. In prior art solutions the dynamic range of the signals is narrowed with logarithm amplifiers. The complexity of logarithm amplifiers, however, causes problems in accuracy and stability. Since the logarithm amplifiers are also expensive, the tendency is to abandon them.
The object of the present invention is thus to provide a method which enables a receiver to operate without logarithm amplifiers, and an A/D conversion to remain linear irrespective of extensive spreading of the power levels of received signals.
The object is achieved with a method described in the preamble, which is characterized in that one or more channel specific signals stronger than the others are searched for, and said channel specific signals stronger than the others are subtracted from a received signal group, in order to equalize the power levels of the signal group, as follows:
the receiver performs at least transfer of the signal group to an intermediate frequency analogously and at least channel filtering digitally, in the channel filtering each signal group being substantially divided into separate channel specific signals of their own, and one or more channel specific signals stronger than the others being D/A converted and subtracted from the intermediate frequency signal group.
A receiver of the invention is characterized in that it comprises means for searching for one or more channel specific signals stronger than the others;
a mixer and a local oscillator arranged to transfer the signal group to the intermediate frequency analogously;
a channel filter arranged to perform the channel filtering digitally, each signal in the signal group being substantially separated as a channel specific signal on a channel of its own;
a D/A converter arranged to D/A convert one or more channel specific signals stronger than the others, and
a connector arranged to subtract one or more D/A converted channel specific signals stronger than the others from the intermediate frequency signal group, in order to equalize the power levels of the signal group.
The method of the invention provides considerable advantages. Expensive, complex and relatively unstable and inaccurate logarithm amplifiers can be abandoned, without impairing the linearity of the A/D conversion.